Heat printers are divided into two types: a thermal printer and a thermal transfer printer. Wherein, the thermal printer generates heat using a print head heater, so as to cause chemical changes of a thermo-sensitive layer on the surface of a printing paper, and thus develop color to form text and images. The thermal transfer printer generates heat using a print head heater to heat the dye on an ink ribbon which is then melted on the surface of a printing paper to form text or images.
FIG. 1 is a schematic view of the arrangement of the printing points of a thermal print head, and as shown therein the thermal print head 10 comprises multiple heaters 20 arranged in a line with an equal distance therebetween, and each heater corresponds to one printing point X1, thereby forming a point line. A print medium 30 moves in the printing direction A, thereby forming point lines such as Y1 and Y2.
The longer the heating time of the print head heater is, the more the heat is and then the higher the printing density is. Therefore, the printing effect of different density levels, i.e., grayscale levels, is obtained by controlling the length of the heating time. Referring to FIG. 2 and FIG. 3, FIG. 2 illustrates a schematic view of a traditional grayscale printing control method; and FIG. 3 is a schematic view of the constitution of the strobe time of the grayscale levels in the method.
This method specifically includes the following steps:    Step S11, the heating time (tu) of a reference grayscale is set;    Step S12, when the print head heater receives data of 1, the heater heats, and when receives data of 0, the heater does not heat.    Step S13, n (n=1, 2, 3 . . . ) pulses are continuously transmitted, and corresponding to each pulse n binary data “1” are transmitted to the heater.
It can be seen that since n pulses need to be continuously transmitted while performing multiple grayscale levels of printing using the above control method, the grayscale levels will not be in direct proportion to the times of printing due to the heat accumulation of the print head, thus resulting in grayscale distortion and then affecting the printing quality.
To solve this problem, a US patent (U.S. Pat. No. 6,798,433), entitled “Method for Increasing Thermal Print Quality”, discloses a control method, and as shown in FIG. 4 and FIG. 5, FIG. 4 is a flow chart of the control method, and FIG. 5 is a schematic view of the constitution of the strobe time of the grayscale levels in the control method.
The control method specifically includes the following steps:    Step S21, the heating time (tu) of a reference grayscale is set;    Step S22, when the print head heater receives data of 1, the heater heats, and when receives data of 0, the heater does not heat.    Step S23, n pulses are transmitted, a time is set for each pulse interval, and corresponding to each pulse n binary data “1” are transmitted to the heater.
It can be seen that the above method reserves enough heat dissipation time for the print head by setting the interval between the transmission of every two pulses, and then avoids the problem in the printing effect of the distortion due to heat accumulation. However, this method also has some defects: relatively slow printing speed, especially for a relatively high grayscale level of printing, the printing speed is affected since a relatively large number of pulses need to be transmitted and the data transmission takes a relatively long time.